Transparent or semi-transparent ceramic orthodontic brackets are more aesthetically appealing than conventional metal brackets. However, one problem with ceramic brackets is that they are generally harder than tooth enamel, which may create tooth wear problems during occlusion.
Transparent or semi-transparent plastic brackets have problems of cracking or deforming when torque is applied to the bracket by rectangular arch wires. Plastic brackets are also generally easily discolored by substances like coffee. However, resins with low absorption coefficients have been developed for use in plastic brackets that inhibit discoloring by substances like coffee.
Plastic brackets having a reinforcement member therein have also been developed as disclosed in U.S. Pat. Nos. 3,930,311; 4,299,569; and 4,302,532. The durability of a plastic bracket can be increased by inserting a metal reinforcement member therein to inhibit cracking or deformation of the bracket.
Known metal reinforcement members for reinforced plastic brackets disclosed in U.S. Pat. No. 3,930,311 are stamped from sheet metal and include a holding notch therein having the same width as the slot of the plastic bracket body. The reinforcement member is inserted in the bracket body such that the holding notch is flush with the surfaces of the slot in the bracket body. An arch wire placed in the slot will thus be in contact with the plastic surfaces of the slot as in unreinforced plastic brackets. Such contact between the arch wire and the plastic surfaces may make movement of the bracket relative to the arch wire difficult and lengthen the time of orthodontic treatment. Another problem in U.S. Pat. Nos. 4,299,569 and 4,302,532 is easy removal of metal reinforcement during clinical application.
The coefficient of static friction between arch wires made from stainless steel or a nickel-titanium alloy and metal surfaces is about 0.15 to 0.2. By contrast, the coefficient of friction between the arch wire and plastic surfaces is about 0.4 to 0.6. Accordingly, smooth movement of the bracket with respect to an arch wire is difficult to achieve in known reinforced plastic brackets.
Furthermore, if known reinforced plastic brackets are to be provided with torque-angulation, a large variety of metal reinforcement members will be needed to provide brackets adapted for use with various types of teeth, orthodontist techniques and individual patient cases as shown in FIGS. 11 and 12 of U.S. Pat. No. 3,930,311. The costs for making such a variety of reinforcement members are high because many metal molds and stamping plates are required.
Moreover, in known reinforced plastic brackets, the inserted metal reinforcement member is designed to be substantially large to reinforce the slot and wing portions of the bracket. The reinforcement member shown in FIGS. 16 and 17 of U.S. Pat. No. 3,930,311 includes a wide base with a plurality of holes. The large size of the reinforcement member may however make the plastic bracket less aesthetically appealing.